Aims: Persistent halogenated organic pollutants (e.g., DDT, PCBs [polychlorinated biphenyls], dioxin) command special attention in public health because they are ubiquitous in the ecosystem and may be causing adverse human health effects at background levels of exposure, via diet. We are conducting several studies of the human health effects of background exposure to organochlorines. Accomplishments: We have completed field work on a case-control study of serum p,p'-DDE (a metabolite of DDT) and PCB levels in relation to risk of breast cancer among African-American women in Los Angeles. Laboratory results are complete (300 cases and 300 controls), and the data are being prepared for analysis and will be ready for analysis in 2000. Our main project is an investigation of health effects of in utero exposure to organochlorine residues among participants in the Collaborative Perinatal Project, for whom a specimen bank (mother's serum) is available. Associations being examined are: 1) mother's p,p'-DDE levels in relation to risk of hypospadias, cryptorchidism, and polythelia in male offspring, 2) mother's PCB level in relation to child's neurodevelopmental outcomes, hearing, and IQ, 3) mother's DDE level in relation to body habitus in adolescence in male offspring. Laboratory analyses are now complete for over 2,500 subjects and the data are being cleaned for analysis, which will begin in 2000. We also examined in utero PCB exposure in relation to neonatal thryoid function in the North Carolina cohort study of effects of early-life organochlorine exposure. We have collaborated with investigators from the Air Force Health Study to examine health effects of dioxin exposure, including diabetes. We are beginning work on a study of the effect of the antiandrogen DDE on anthropometric measures at birth among neonates from a highly exposed population in Mexico.